1. Field of the Invention
The invention relates to a transmission system comprising a transmitter for convening an input signal into a transmit signal, and a channel for transporting the transmit signal to a receiver. The receiver comprises polarization adapting means for deriving from a received signal a further signal which has a state of polarization which is variable in response to a control signal, supplied to the polarization adapting means. The transmission system also comprises means for effecting polarization variations in the further signal produced by the polarization adapting means, and the receiver further comprises means for measuring an error signal corresponding to the difference between the actual state of polarization and the desired state of polarization of the further signal, and control means for deriving the control signal from the error signal.
The invention likewise relates to a receiver for such a transmission system.
2. Description of the Related Art
A transmission system as defined in the opening paragraph is known from the journal article "Comparison of Polarization Handling Methods in Coherent Optical Systems" by R. Noe et al. in IEEE Journal of Lightwave Technology, Vol. 9, No. 10, October 1991.
For transporting an input signal through a channel, the signal originating from a signal source is amplitude, frequency or phase modulated by the input signal in the transmitter. The transmit signal thus obtained is transported to the receiver by the optical channel. This transmission system may be an optical transmission system, but it may also be a radio transmission system. An optical channel may comprise, for example, a glass fibre, but the channel may also comprise a direct-sight link through free space. In radio systems the channel is formed by free space.
Various types of receivers are possible for receiving the signal transmitted by the transmitter. In a number of these receivers, the received signal, or an auxiliary signal necessary for the demodulation, has certain polarization properties. Examples of these receivers are receivers utilizing polarization-sensitive components such as, for example, aerials, optical amplifiers or optical filters. Other polarization-sensitive receivers are the coherent (optical) receivers such as heterodyne, homodyne and phase-diversity receivers.
Generally, the polarization properties, for example, the direction of polarization, of the received optical signal are indefinite and, furthermore, not constant with time. Without precautionary measures the amplitude of the demodulated signal may vary between a maximum value (when the actual state of polarization corresponds to the desired state of polarization) and a minimum value (when the actual state of polarization is orthogonal relative to the desired state of polarization).
An example of a polarization-sensitive component is a laser amplifier for amplifying the received signal. The gain factor of such a laser amplifier often depends on the state of polarization of the received optical signal. For achieving a maximum gain factor, the state of polarization of the received signal is changed by polarization adapting means, so that this state of polarization corresponds to the desired state of polarization. The further signal is then the received signal whose state of polarization has changed.
In a coherent optical receiver such as, for example, a heterodyne receiver, the optical signal having a very high frequency (for example, 10.sup.14 Hz) is converted into a signal having a much lower frequency (for example, 10.sup.9 Hz). For this purpose, an optical directional coupler and an optoelectric converter mix the received optical signal with an optical signal coming from a local laser. As a result, an intermediate frequency signal is obtained which has a frequency that is equal to the difference between the frequency of the received light signal and that of the optical signal coming from the local laser.
In order to let this mixing process be attended with the least possible signal loss, it is necessary that the state of polarization of the received optical signal and the state of polarization of the optical signal coming from the local laser be the same. This may be achieved in that the polarization adapting means adapt the state of polarization of the received optical signal or the state of polarization of the optical signal generated by the local laser. The further optical signal is in this case the intermediate frequency signal obtained from the combination of the received optical signal and the locally generated signal.
For detecting an error signal which is a measure of the difference between the actual state of polarization and the desired state of polarization, (minor) polarization variations are effected in the output signal of the polarization adapting means. If the output signal of the receiver becomes larger when polarization variations in a specific direction are made, this means that the state of polarization is to be adapted more in this direction. On the other hand, if the output signal of the receiver becomes smaller when these polarization variations are made, the state of polarization is to be adapted in another direction. If no change in the output signal of the receiver occurs, that shows there is a (substantially) optimum polarization of the further signal. Instead of effecting polarization variations in the receiver, it is likewise conceivable that they are already effected in the transmitter.
In a transmission system as defined in the opening paragraph it appears that the operation of the polarization adapting means may be disturbed by undesired variations in the receiver input signal, or (in the case of a coherent receiver) in the signal coming from the local laser. These undesired variations may be, for example, amplitude variations or polarization variations of the input signal.